Common Types of Corrosion and Their Impact

Definition of Corrosion

Corrosion is the gradual degradation or destruction of materials, typically metals, through reactions with their environment. This process leads to the weakening, reduction and eventual failure of the material. The most common example is the rusting of iron, where iron reacts with oxygen and water to form iron oxide, a brittle and flaky substance that compromises the structural integrity of the metal. Other metals, such as aluminum, copper, and stainless steel, also experience similar reactions, though the specific compounds formed can vary.

Factors That Influence Corrosion

Several factors can influence the rate and severity of corrosion. Understanding these factors can help in developing effective strategies for preventing and managing material failure.

• Material Composition: Different metals and alloys have varying levels of corrosion resistance. For example, stainless steel contains chromium, which forms a passive oxide layer that protects against degradation. In contrast, carbon steel lacks this protective layer and degrades more easily. The specific alloying elements and their proportions can significantly impact a material’s susceptibility.
• Environmental Conditions: Temperature, humidity, and the presence of salts or pollutants can accelerate the rate of corrosion. High temperatures can increase the rate of chemical reactions, while high humidity provides the moisture necessary for many types of material failure. Salts, particularly sodium chloride from seawater or road deicing, are highly aggressive as they facilitate the formation of electrolytes that promote reactions.
• Surface Condition: The condition of a material’s surface plays a crucial role in initiation. Rough surfaces and imperfections, such as scratches or pits, can trap moisture and corrosive agents, creating localized areas where the process can begin. Smooth, well-maintained surfaces are less likely to experience severe issues.
• Electrochemical Potential: Differences in electrochemical potential between metals can create galvanic cells that accelerate material degradation. When two different metals are in electrical contact in the presence of an electrolyte, the more anodic metal will degrade faster, while the more cathodic metal will be protected. This phenomenon is known as galvanic corrosion and is a common issue in mixed-metal structures and components.

Types of Corrosion

Material corrosion manifests in various forms, each with distinct characteristics and mechanisms. Understanding the different types is essential for identifying specific challenges and applying appropriate prevention and mitigation strategies. Below are some of the major types, each affecting materials in unique ways.

Uniform Corrosion

Uniform corrosion, also known as general corrosion, is the most common form. It occurs uniformly across the entire surface of the metal, leading to a consistent and even loss of material over time. This type of corrosion is typically characterized by a uniform thinning of the metal, which can be easily predicted and managed through regular maintenance and inspection. Uniform corrosion is caused by a chemical or electrochemical reaction between the metal and its environment, such as exposure to air, moisture, or chemical agents. While it can be less dangerous than localized corrosion like pitting or crevice corrosion, uniform corrosion still requires careful monitoring and prevention strategies to avoid significant material loss and structural failure over time.

Pitting Corrosion

Pitting is a localized form of corrosion that leads to the formation of small pits or holes on the metal surface. This type is particularly insidious because it can cause significant corrosion damage, even if only a small area is affected.

Pitting typically occurs in materials that are otherwise resistant to degradation, such as stainless steel. It is often caused by localized breakdown of the protective oxide layer on the material’s surface, which can be triggered by chloride ions found in environments such as seawater or deicing salts. Once the oxide layer is compromised, the exposed metal becomes susceptible to rapid localized attack.

The impact can be severe, as the pits can penetrate deep into the material, leading to structural weaknesses and potential failure. Because pitting is often difficult to detect and measure, it poses a significant risk in applications where material integrity is critical.

Crevice Corrosion

Crevice degradation occurs in confined spaces where the local environment differs significantly from the exposed surface. These confined spaces, or crevices, can be found in joints, overlaps, under gaskets, washers, or deposits of dirt and biofilms.

The main characteristic is that it is driven by a difference in oxygen concentration between the interior of the crevice and the external environment. The restricted flow of oxygen inside the crevice leads to a localized depletion of oxygen, creating an electrochemical gradient. This oxygen-deprived area becomes anodic and begins to degrade, while the more oxygen-rich external surface remains cathodic.

This type can be particularly aggressive because the confined space traps corrosive agents such as chloride ions, which can further accelerate the degradation process. Crevice corrosion can lead to significant material loss and structural failure if not detected and managed properly.

Intergranular Corrosion

Intergranular corrosion or integranual attack occurs along the grain boundaries of a material. These boundaries are the interfaces where crystals of different orientations meet within the metal. Intergranular attack can cause the material to lose its structural integrity without significant overall material loss, leading to potential catastrophic failure.

This type of corrosion is often caused by certain manufacturing processes or the presence of impurities. For example, during welding or heat treatment, the temperature can cause certain elements, such as chromium in stainless steel, to migrate and form compounds like chromium carbides at the grain boundaries. This process depletes the surrounding areas of chromium, reducing their resistance and making the grain boundaries susceptible to attack. Additionally, impurities such as sulfur, phosphorus, or lead can segregate at the grain boundaries, promoting degradation.

Intergranular degradation can be particularly problematic in high-stress applications, as it can lead to unexpected and sudden failures due to the material weakening along these critical boundaries.

Stress Corrosion Cracking (SCC)

Stress corrosion cracking (SCC) is a process that involves the combined action of tensile stress and a corrosive environment, leading to the formation of cracks in a material. This type of corrosion is particularly dangerous because it can occur at stress levels well below the material’s typical tensile strength, making it hard to predict and prevent.

SCC usually starts with small cracks that form on the material’s surface and then propagate inward, driven by the tensile stress. These cracks can grow rapidly, leading to sudden and catastrophic failure of the material. The electrochemical reaction in the corrosive environment further accelerates the crack propagation. SCC is commonly observed in metals such as stainless steel, aluminum, and certain high-strength alloys when exposed to specific corrosive environments, such as chlorides or caustic solutions.

The primary factors that contribute to SCC include the material’s susceptibility, the presence of tensile stress (either residual or applied), and a corrosive environment that promotes crack initiation and propagation.

Galvanic Corrosion